Actually, when I think of it a little further, it was explained completely.
OK, here's what we know. When Revan and Malak found the first Star Map on Dantooine, they, or rather Revan (Malak simply followed him), were drawn by it and decided to find the missing pieces of the puzzle. At the end of the road was the Star Forge. This isn't emphasised in the game, but it seems pretty obvious that they discovered that Star Map before they went to fight in the Mandalorian Wars, since they didn't return to the Academy after they went to war. It also seems obvious that finding the Star Maps was the real reason why Revan left to fight in the war.
Now, consider this:
- It was said in the game that Revan took his fleet to the Unknown Regions, after they defeated the Mandalorians and it was said that something happened to him there that turned him to the Dark Side.
- We know that he found the Star Forge and used it.
- The Star Forge is in the Unknown Regions.
- The Star Forge is a tool, for lack of a better term, of the Dark Side.
I think that the conclusion you can draw from all this is pretty obvious - When Revan found the Star Forge he completely turned to the Dark Side and Malak along with him. They were already tainted when they found that first Star Map on Dantooine.
I know what you're going to ask: Where did that taint come from? And here's the answer: both of them, especially Revan, were refered to as being arrogant and overconfident and who knows what else (mostly by Vrook, if I remember correctly) and when it comes to Jedi, those traits come with a price - the possibility of falling to the Dark Side.